1. Field of the Invention
The present invention relates to a method of producing a vinyl chloride-based polymer using suspension polymerization, in which the heat removal efficiency can be improved without affecting the quality of the product polymer.
2. Description of the Prior Art
In the production of vinyl chloride-based polymers, large scale polymerization vessels and reduced polymerization times are used to improve the productivity. In one technique, a method is employed in which heat removal is performed using both a polymerization vessel jacket and a reflux condenser, in order to increase the reaction heat removal capability per unit of time. Particularly in cases in which the maximum value of the total heat removed per unit of time exceeds 10,000 MJ/hr, it is typical for a coolant which has been cooled using a cooling apparatus such as a refrigeration device to be used as a cooling medium and passed through the polymerization vessel jacket, and for a reflux condenser to also be used. In these types of methods requiring the use of a coolant, from an economic viewpoint it is desirable to reduce the proportion of heat removal performed by the jacket, in order to economize on the power costs associated with operating the refrigeration device. As a result, it is necessary to increase the proportion of heat removal performed by the condenser, without requiring the use of the coolant cooled by the refrigeration device.
However, in those cases in which a vinyl chloride-based polymer is produced by a suspension polymerization method using a dispersant (such as a partially saponified polyvinyl alcohol, or a cellulose ether), if heat removal is conducted so that the proportion of the total heat removal performed by the reflux condenser per unit of time exceeds 30%, then not only are the particle size distribution, the bulk specific gravity and the porosity of the product polymer all affected, but foaming of the polymerization reaction liquid increases during the polymerization, and in some cases the polymerization reaction liquid may erupt up into the reflux condenser causing particles of polymer to become deposited on the inside of the condenser, and these particles may then fall back into the polymerization reaction liquid, causing an increase in fish eyes and defects within the molded polymer film. Accordingly, in order to prevent these types of problems, the quantity of heat removed by the reflux condenser per unit of time during the polymerization reaction is typically restricted to less than 30% of the total quantity of heat removed.
An object of the present invention is to provide a method of producing a vinyl chloride-based polymer by suspension polymerization using a polymerization vessel equipped with both a reflux condenser and a jacket, wherein the proportion of the total heat removal performed by the reflux condenser per unit of time can be increased, and the proportion of the total heat removal performed by the jacket can be reduced, without affecting the quality of the product polymer.
The present invention provides a method of producing a vinyl chloride-based polymer by suspension polymerization of vinyl chloride or a monomer mixture comprised of vinyl chloride using a polymerization vessel equipped with a reflux condenser and a jacket, in which a maximum value of total quantity of heat removed per unit of time is at least 10,000 MJ/hr, said method comprising:
(1) commencing heat removal when or after a polymerization conversion rate reaches 15%,
(2) controlling said reflux condenser so as to remove a fixed quantity of heat per unit of time during a period from a point where said polymerization conversion rate reaches a preset polymerization conversion rate within a range from 20 to 35%, until a point where said polymerization conversion rate reaches another preset polymerization conversion rate within a range from 50 to 65%, wherein a ratio of a quantity of heat removed by said reflux condenser (A MJ/hr) relative to a total quantity of heat removed, per unit of time, is within a range from 30 to 60%,
(3) controlling said reflux condenser so as to remove a fixed quantity of heat per unit of time during a period from a point where said polymerization conversion rate reaches a preset polymerization conversion rate within a range from 70 to 75%, until a point where said polymerization conversion rate reaches another preset polymerization conversion rate of at least 80%, wherein a ratio of a quantity of heat removed by said reflux condenser (B MJ/hr) relative to a total quantity of heat removed, per unit of time, is within a range from 20 to 30%,
wherein a ratio of said quantity of heat removed (A MJ/hr)/said quantity of heat removed (B MJ/hr) is in a range from 1.2 to 2.0.
As follows is a more detailed description of the present invention.
 less than Heat Removal Pattern Using a Reflux Condenser greater than 
As a result of intensive investigations, the inventors of the present invention discovered that when a polymerization reaction is conducted using a reflux condenser, during the period when the polymerization conversion rate is between 20 and 65%, even if the quantity of heat removed by the reflux condenser per unit of time (under a fixed rate operation) exceeds 30% of the total heat generated by the polymerization per unit of time, little foaming occurs in the polymerization reaction liquid, and effects on the quality of the polymer such as the bulk specific gravity, the porosity and the occurrence of fish eyes are minimal.
In other words, in a reflux condenser heat removal pattern according to the present invention, heat removal using the reflux condenser is commenced at or after the point where the polymerization conversion rate reaches 15%, and then fixed rate heat removal operation of the reflux condenser is performed over two subsequent stages, during a preset range (a first stage) when the polymerization conversion rate is between 20 and 65%, and during a preset range (a second stage) when the polymerization conversion rate exceeds 70% (and is preferably between 70 and 90%), and by setting the quantity of heat removed by the reflux condenser during the first stage of the heat removal pattern to a larger value than that of the second stage, the quantity of coolant required for the heat removal performed by the polymerization vessel jacket can be reduced, thereby conserving energy by reducing the operational load on the refrigeration device, while the quality of the product vinyl chloride-based polymer remains unaffected.
The polymerization vessel used in the present invention is a jacketed vessel with a reflux condenser attached either directly or indirectly at the top of the vessel. There are no particular restrictions on the structure of the polymerization vessel jacket, and known systems can be used. Furthermore, in the case of a large scale polymerization vessel, the vessel may also include a cooling baffle or a cooling coil or the like inside the vessel in order to increase the area of heat removal surfaces. A stirrer is provided either through the upper portion of the vessel or in the base of the vessel.
In a heat removal pattern of the present invention, initial commencement of heat removal using the reflux condenser must occur when or after the polymerization conversion rate has reached 15%. If heat removal is commenced before the polymerization conversion rate has reached this level, then heat removal starts at a stage in which the apparent particle size distribution is yet to be determined, and consequently the quantity of refluxing monomers is large, and the distribution within the suspension system becomes very non-uniform, making control of the particle size distribution of the product polymer very difficult. In addition, dispersion of the catalyst also becomes non-uniform, resulting in an unfavorable increase in the occurrence of fish eyes.
Furthermore, if the heat removal quantity is raised instantaneously from the commencement of heat removal to the fixed rate of the aforementioned first stage, then a large amount of foaming occurs very rapidly, resulting in fish eyes and other defects, and consequently a fixed period of time is provided prior to commencing the fixed rate heat removal operation of the reflux condenser of the first stage, and during this fixed period, the heat removal quantity is preferably raised gradually until it reaches the fixed rate of the first stage.
In the present invention, during the fixed rate heat removal operation of the first stage, the ratio of the quantity of heat removed by the reflux condenser relative to the total quantity of heat removed per unit of time must be controlled within a range from 30 to 60%. If this heat removal ratio is less than 30%, then the effects of the present invention in reducing the quantity of coolant required for the heat removal performed by the polymerization vessel jacket, and reducing the operational load on the refrigeration device cannot be achieved. In contrast, if the heat removal ratio exceeds 60%, then as described above in relation to the conventional technology, problems arise in terms of the quality of the product polymer, including an increase in the occurrence of fish eyes in the molded film arising from foaming within the polymerization liquid.
On the other hand, during the fixed rate heat removal operation of the second stage, the ratio of the quantity of heat removed by the reflux condenser relative to the total quantity of heat removed per unit of time must be controlled within a range from 20 to 30%. If this heat removal ratio exceeds 30%, then in this second stage reflux condenser heat removal stage, during the latter stages of the polymerization, namely at polymerization conversion rates of at least 70% (and preferably for polymerization conversion rates from 70 to 90%), volumetric shrinkage accompanying the progress of the reaction causes a lowering of the reaction liquid level, leading to an increase in the apparent slurry concentration, that is, an increase in viscosity, and consequently eruptions from the slurry are more likely. As a result, the ratio during this stage must be maintained at no more than 30%, as in the conventional technology. In contrast if the heat removal ratio falls below 20%, then the effect of improving the heat removal efficiency using the reflux condenser is weakened.
Furthermore in the present invention, if the quantity of heat removed by the reflux condenser per unit of time during the first stage fixed rate heat removal operation is termed A MJ/hr, and the quantity of heat removed by the reflux condenser per unit of time during the second stage fixed rate heat removal operation is termed B MJ/hr, then the ratio A/B must be within a range from 1.2 to 2.0. If the quantity of heat removed by the reflux condenser during the first stage is less than 1.2 fold the quantity of heat removed during the second stage, then only a small energy conservation effect is achieved as a result of the reduction in the operational load on the refrigeration device arising from having increased the heat removal quantity performed by the reflux condenser to a higher value than conventional technology. On the other hand, if the above ratio exceeds 2.0, then quality control of factors such as the particle size distribution within the product polymer becomes difficult. By setting the heat removal ratio for the first and second stages as described above, then for example in the case in which the polymerization time is shortened for a polymerization conducted in a large scale polymerization vessel exceeding 100 m3, the energy conservation effect described above is marked.
Furthermore, during the heat removal of the first stage of a heat removal pattern according to the present invention, a heat removal control method using a liquid level gauge as disclosed in Japanese Laid-open Patent publication (kokai) No. 7-25909 (JP7-25909A), or conventional antifoaming agents may also be used. In addition, if necessary, a method in which a mixed gas of the vinyl chloride monomer and an inert gas such as nitrogen gas or carbon dioxide is discharged externally (inert purge) may also be performed concurrently.
 less than Heat Removal From the Polymerization Vessel greater than 
Prior to introducing the raw materials for the polymerization into the polymerization vessel, and during the supply of these raw materials, the jacket and the baffle are held at room temperature, and the reflux condenser is held at a temperature of at least 70xc2x0 C., and preferably at a temperature within a range from 70 to 90xc2x0 C. Following completion of the introduction of the polymerization raw materials into the polymerization vessel, hot water is passed through the jacket and the baffle, and the temperature of the reactant mixture is raised. When the temperature of the reactant mixture has reached at the set polymerization temperature, cold water is passed through the jacket and the baffle, and the temperature of the reactant mixture is maintained at the set polymerization temperature while the polymerization reaction proceeds.
When or after the polymerization conversion rate reaches 15%, cold water is passed through the reflux condenser, and heat removal using the reflux condenser is commenced. Once heat removal using the reflux condenser has commenced, the quantity of heat removed by the reflux condenser is gradually increased, and when the quantity of heat removed by the reflux condenser per unit of time reaches a preset value at a polymerization conversion rate within a range from 20 to 35%, fixed rate heat removal operation of the reflux condenser is commenced, and is continued until the polymerization conversion rate reaches a value within a range from 50 to 65% (the first stage fixed rate heat removal operation).
During this fixed rate heat removal operation, the ratio of the quantity of heat removed by the reflux condenser (A MJ/hr) relative to the total quantity of heat removed per unit of time is within a range from 30 to 60%, and the heat removal operation is performed with the reaction mixture maintained at the set polymerization temperature.
After the first stage fixed rate heat removal operation is continued until the polymerization conversion rate reaches a value within a range from 50 to 65%, the quantity of heat removed by the reflux condenser is reduced, and when the quantity of heat removed by the reflux condenser per unit of time reaches a preset value, at a polymerization conversion rate within a range from 70 to 75%, fixed rate heat removal operation of the reflux condenser is recommenced, and is continued until the polymerization conversion rate reaches a value exceeding 80% (and preferably a value within a range from 80 to 90%) (the second stage fixed rate heat removal operation).
During this second fixed rate heat removal operation, the ratio of the quantity of heat removed by the reflux condenser (B MJ/hr) relative to the total quantity of heat removed per unit of time is within a range from 20 to 30%, and the heat removal operation is performed with the reaction mixture maintained at the set polymerization temperature.
The first stage fixed rate heat removal operation and the second stage fixed rate heat removal operation are conducted so that the ratio of the quantity of heat removed by the reflux condenser (A MJ/hr)/the quantity of heat removed by the reflux condenser (B MJ/hr)=1.2 to 2.0.
The polymerization conversion rate during the polymerization reaction is calculated according to the following formula.                     Polymerization        ⁢                  xe2x80x83                ⁢                  conversion                ⁢                  xe2x80x83                ⁢                  rate                ⁢                  xe2x80x83                ⁢                  (          %          )                    =                        total                ⁢                  xe2x80x83                ⁢                  quantity                ⁢                  xe2x80x83                ⁢                  of                ⁢                  xe2x80x83                ⁢                  heat                ⁢                  xe2x80x83                ⁢                  removed                ⁢                  xe2x80x83                ⁢                              (            kcal            )                    /          X                      ⁢          xe2x80x83                                                X            =                        ⁢                                                            quantity                                ⁢                                  xe2x80x83                                ⁢                                  of                                ⁢                                  xe2x80x83                                ⁢                                  charged                                ⁢                                  xe2x80x83                                ⁢                                  vinyl                                ⁢                                  xe2x80x83                                ⁢                                  chloride                                ⁢                                  xe2x80x83                                ⁢                                  (                                      kg                                    )                                xc3x97                368                ⁢                                  xe2x80x83                                ⁢                                  (                                      kcal                    ⁢                                          /                                        ⁢                                          kg                                                        )                                            +                                                                                    ⁢                                          quantity                            ⁢                              xe2x80x83                            ⁢                              of                            ⁢                              xe2x80x83                            ⁢                              charged                            ⁢                              xe2x80x83                            ⁢                                                C                  0                                ⁡                                  (                                      kg                                    )                                            xc3x97                              C                1                            ⁢                              (                            ⁢              kcal              ⁢                              /                            ⁢                              kg                            ⁢                              )                                                          ⁢          xe2x80x83      
(wherein, C0 represents a comonomer, and C1 represents the reaction calorific value of the comonomer)
Total quantity of heat removed (kcal)={(total heat removed by jacket+buffle (kcal))+total heat removed by reflux condenser (kcal)}
The value of xe2x80x9ctotal heat removed by jacket+baffle (kcal)xe2x80x9d described above is determined every two minutes after the passage of coolant through the jacket and the baffle is commenced, by calculating the quantity of heat removed during the just completed two minute period using the formula below, and then determining the total sum of these two minute heat removal quantities.
2 minute heat removal quantity (kcal)=R01(kg/hr)xc3x97(T02xe2x88x92T01)xc2x0 C.xc3x97D{kcal/(kgxc2x7xc2x0 C.)}xc3x97(2 minutes/60 minutes) 
(wherein in the above formula,
R01: coolant flow rate through the jacket and the baffle (kg/hr)
T02: exit temperature of coolant from the jacket and the baffle (xc2x0 C.)
T01: entry temperature of coolant to the jacket and the baffle (xc2x0 C.)
D: specific heat of the coolant)
Furthermore, the value of xe2x80x9ctotal heat removed by reflux condenser (kcal)xe2x80x9d described above is also determined every two minutes in a similar manner to that described above, by calculating the quantity of heat removed during the just completed two minute period, and then determining the total sum of these two minute heat removal quantities.
2 minute heat removal quantity (kcal)=R11 (kg/hr)xc3x97(T12xe2x88x92T11)xc2x0 C.xc3x97D{kcal(kgxc2x7xc2x0 C.)}xc3x97(2 minutes/60 minutes) 
(wherein in the above formula,
R11: coolant flow rate through the reflux condenser (kg/hr)
T12: exit temperature of coolant from the reflux condenser (xc2x0 C.)
T11: entry temperature of coolant to the reflux condenser (xc2x0 C.)
D: specific heat of the coolant)
 less than Monomer Raw Material greater than 
The monomer raw material used in the present invention is either vinyl chloride or a monomer mixture comprising vinyl chloride as the primary constituent. A monomer mixture comprising vinyl chloride as the primary constituent comprises at least 50% by weight of vinyl chloride, as well as at least one other monomer which is copolymerizable with vinyl chloride. Examples of the other monomer which is copolymerizable with vinyl chloride include vinyl esters such as vinyl acetate and vinyl propionate; acrylate esters or methacrylate esters such as methyl acrylate and ethyl acrylate; olefins such as ethylene and propylene; as well as maleic anhydride, acrylonitrile, styrene and vinylidene chloride. These monomers may be used singularly, or in combinations of two or more monomers.
 less than Dispersant greater than 
In a method of the present invention, for the case in which either vinyl chloride or a monomer mixture comprising vinyl chloride as the primary constituent undergoes suspension polymerization in an aqueous medium, there are no particular restrictions on the dispersant used, and the types of dispersants used in conventional vinyl chloride-based polymer production are suitable.
Specific examples of these types of dispersants include water soluble cellulose ether compounds such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose and hydroxypropylmethyl cellulose; partially saponified polyvinyl alcohols such as water soluble partially saponified polyvinyl alcohol; acrylic acid polymers; water soluble polymers such as gelatin; oil soluble emulsifiers such as sorbitan monolaurate, sorbitan trioleate, glycerin tristearate, and block copolymers of ethylene oxide and propylene oxide; and water soluble emulsifiers such as polyoxyethylene sorbitan monolaurate, polyoxyethylene glycerin oleate and sodium laurate. These dispersants may be used singularly, or in combinations of two or more different dispersants. In the example described above, water soluble cellulose ethers and partially saponified polyvinyl alcohols are particularly effective. The quantity of dispersant added is typically within a range from 0.02 to 1 part by weight per 100 parts by weight of the raw material monomer.
 less than Polymerization Initiator greater than 
There are no particular restrictions on the polymerization initiator used in the method of the present invention, and the types of initiators used in conventional vinyl chloride-based polymer production are suitable.
Specific examples of suitable polymerization initiators include peroxycarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate and diethoxyethyl peroxydicarbonate; peroxy ester compounds such as t-butyl peroxypivalate, t-hexyl peroxypivalate, t-butyl peroxyneodecanoate and xcex1-cumyl peroxyneodecanoate; peroxides such as acetyl cyclohexylsulfonyl peroxide, 2,4,4-trimethylpentyl-2-peroxyphenoxy acetate and 3,5,5-trimethylhexanoyl peroxide; azo compounds such as azobis-2,4-dimethylvaleronitrile and azobis(4-methoxy-2,4-dimethylvaleronitrile); as well as potassium persulfate, ammonium persulfate and hydrogen peroxide and the like. These polymerization initiators may be used singularly, or in combinations of two or more different initiators. The quantity of polymerization initiator added is typically within a range from 0.02 to 0.3 parts by weight per 100 parts by weight of the raw material monomer.
 less than Antioxidants greater than 
There are no particular restrictions on the antioxidants used in the present invention, and the types of antioxidants typically used in conventional vinyl chloride-based polymer production are suitable.
Specific examples of suitable antioxidants include phenol compounds such as 2,2-di(4xe2x80x2-hydroxyphenyl)propane, hydroquinone, p-methoxyphenol, t-butylhydroxyanisol, n-octadecyl-3-(4-hydroxy-3,5-di-t-butylphenyl)propionate, t-butylhydroquinone, 2,5-di-t-butylhydroquinone, 4,4xe2x80x2-butylidenebis(3-metyl-6-t-butylphenol), 3,5-di-t-butyl-4-hydroxytoluene, 2,2xe2x80x2-methylene-bis(4-ethyl-6-t-butylphenol), triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,6-di-t-butyl-4-sec-butylphenol, 2,6-di-t-butyl-4-methylphenol, t-butylcatechol, 4,4xe2x80x2-thiobis(6-t-butyl-m-cresol), tocopherol and nordihydro-guaiaretic acid; semicarbazide derivatives such as semicarbazide, 1-acetylsemicarbazide, 1-chloroacetylsemicarbazide, 1-dichloroacetylsemicarbazide, 1-benzoylsemicarbazide and semicarbazone; thiocarbazide derivatives such as carbohydrazide, thiosemicarbazide and thiosemicarbazone; amine compounds such as phenylnaphthylamine, N,Nxe2x80x2-diphenyl-p-phenylenediamine and 4,4xe2x80x2-bis(dimethylbenzyl)diphenylamine; nitro and nitroso compounds such as nitroanisol, N-nitrosodiphenylamine, nitroaniline and the aluminum salt of N-nitrosophenylhydroxylamine; phosphorus compounds such as triphenyl phosphite, diphenylisodecyl phosphite, phenyldiisodecyl phosphite, 4,4xe2x80x2-butylidene-bis(3-methyl-6-t-butylphenyl-di-tridecyl) phosphite, cyclic isopentane tetraylbis(octadecyl phosphite), tris(nonylphenyl) phosphite and tris(dinonylphenyl) phosphite; unsaturated hydrocarbon compounds such as styrene, 1,3-hexadiene and methylstyrene; and sulfur compounds such as dilauryl thiodipropionate, dimyristyl thiodipropionate, distearyl thiodipropionate, dodecylmercaptan and 1,3-diphenyl-2-thiourea. These antioxidants can be used singularly, or in combinations of two or more compounds.
Of the above antioxidants, from the viewpoints of limiting initial coloration when the product polymer is molded into a film or the like, and limiting scale adhesion to the polymerization vessel, 3,5-di-t-butyl-4-hydroxytoluene, triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], t-butylhydroxyanisol, t-butylhydroquinone, 2,6-di-t-butyl-4-sec-butylphenol and octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate are preferred. The quantity of antioxidants added is typically within a range from 0.0001 to 0.1 parts by weight per 100 parts by weight of the raw material monomer.
 less than Other Conditions greater than 
Other conditions associated with the polymerization, such as the method of supplying the aqueous medium, the vinyl chloride or the monomer mixture comprising vinyl chloride, the dispersant and the polymerization initiator and the like to the polymerization vessel, the relative proportions within the reaction mixture and the polymerization temperature may be similar to conventional methods.
Moreover in the method of the present invention, where necessary, other additives typically used in the production of vinyl chloride-based polymers such as polymerization degree regulators, chain transfer agents, antistatic agents, and scale adhesion prevention agents may also be added to the polymerization system. Furthermore, the antioxidant can be added to the polymerization system prior to commencement of the polymerization reaction, during the polymerization or following completion of the polymerization, in order to control the polymerization reaction and prevent deterioration of the product polymer.